That is the ground for the O2 sensor heater. Just bolt it down to any clean spot on the block, head or intake manifold.
While you are bolting down grounds, check the engine block to chassis ground. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.
From the factory it is bolted to the back of the head along with the ground strap for the firewall. But you can bolt it to the intake if desired like jrichker said.